We added the SkinCodexThemes attribute to allow skins to choose which Codex theme they wanted to use (wikimedia-ui or wikimedia-ui-legacy). We later added experimental as a third theme. However, we then got rid of the legacy theme, and made the experimental theme the default one. In the code that interprets this attribute, all three themes now point to the same files, and no skin sets this attribute anymore.

If we think we won't introduce new themes in the future, we should consider removing this attribute.